This invention relates to hot melt composition coatings that enable paper products, more particularly, paper, paperboard, and corrugated paper containers to be repulpable and recyclable under normal repulping temperature and pH conditions when the products are treated with such hot melt compositions, but without sacrificing the moisture/water resistance of the coated products.
For years, hot melt coating compositions have been used to treat paper and paperboard products, which are then used as wrapping or packaging for moist, wet or fatty foods. Typical examples are the hot melt composition coated corrugated paperboard containers. These water resistant containers are used to ship and store fresh, perishable foods such as poultry, fish and meat. Often containers are filled with ice. The hot melt coating composition imparts water and moisture resistance to the containers.
Currently, hot melt treated fibrous products are not considered recyclable or repulpable under normal conditions using conventional repulping equipment and techniques. They are either burnt as an energy source or landfilled. Considering the large quantity of hot melt treated fibrous products and ecological concerns focused on the disposal of used products, it is highly desirable to discover a way so that hot melt coating compositions can be applied using conventional equipment and the treated paper, paperboard and container can be repulped and recycled under normal conditions. Most importantly, the discoveries should not sacrifice the performance of the prior art coating compositions and should remain competitive in cost.
U.S. Pat. No. 2,984,585 to Sherman is representative of these prior art coatings for paper or paper like materials which provide a barrier resistance to water vapor. In this patent, a combination of polyethylene and wax is used as a coating, the polyethylene being applied as a first coating followed by an application of a wax coating on top of the polyethylene.
U.S. Pat. No. 3,629,171 to Kremer et al. is another patent disclosing wax compositions used in curtain coating boards for food packaging applications.
While the coatings disclosed in the Sherman and Kremer et al. patents may provide moisture resistance, articles with these types of coatings are difficult if not impossible to repulp and/or recycle since the wax-containing coating cannot be separated from the paper or paper-like substrate.
In view of the ecological concerns discussed above and the difficulty with recycling or repulping certain prior art hot melt treated fibrous products, the prior art has proposed modified hot melt composition coatings to achieve both recyclability and/or repulpability.
International Publication No. WO9503360 to Sandvick et al. is concerned with a repulpable hot melt polymer/fatty acid composition for coating paper and paperboard which makes the paper products more water resistant. While paper and paperboard coated with these types of compositions may be more repulpable, they are not without their disadvantages. In general, the added fatty acid reduces the coefficient of friction of the recycled fibers, promotes undesirable oxidation of the paraffin wax component of the composition and also promotes increased coating equipment corrosion. When using fatty acids, it is not uncommon to require the use of stainless steel equipment for handling the coating composition. For repulpability, fatty acid soaps must be formed through the use of alkaline pH conditions in the repulping step, these conditions further complicating the processing equipment requirements.
International Publication No. WO9418272 to Lan also discloses a fatty acid-containing composition for wax based coating compositions to produce recyclable paper board containers. In these compositions, a fatty acid content of 10 to 30% by weight is added to the wax compositions to provide repulpability. These types of compositions are also disadvantageous for the same reasons set forth above with respect to the Sandvick et al. patent.
International Publication No. WO9105107 to Michelman is concerned with a hot melt wax coating composition for yielding recyclable paper products wherein the wax contains a non-ionic, anionic or cationic dispersant chemical (in the form of a surfactant). The surfactant is employed in these wax compositions at levels to enhance repulpability through improved wax dispersion. The addition of surfactants to wax coatings creates a number of problems, including significant cost increases, lowering the moisture resistance properties of the wax coating, primarily since the surfactants migrate to the wax coating surface and, finally, reducing the coefficient of friction of the recycled fibers.
In U.S. Pat. No. 3,607,348 to Wray et al., a double coated paper product is disclosed suitable for printing and coating with carbon transfer inks. In this paper product, a hydrophobic first coating comprising a thermoplastic material similar to wax is applied followed by a hydrophilic second coating thereon. The hydrophobic coating comprises clay and microcrystalline waxes wherein the clay loading is about 45% by weight. According to this patent, the second coating on the surface of the paper product is an aqueous-based hydrophilic coating comprising 2% by weight of sodium alginate and 0.4% of a wetting agent and water. The coatings of Wray et al. are designed to deliver printability rather than moisture barrier resistance and paraffin waxes are not used. Wray et al. also fail to recognize any recyclability or repulpability characteristics.
U.S. Pat. No. 3,723,169 to Guastella et al. describes a process for coating paper that applies a hot-melt coating. The hot-melt coating is extruded and contains as essential ingredients a heat extrudable binder and a coating pigment, e.g., clay. The heat extrudable binder is preferably polyethylene or a mixture of polyethylene and ethylene vinyl acetate. Polyethylene does not have the same barrier properties as paraffin waxes and has a melting point far too high for conventional repulping processes. The hot melt composition of Guastella et al. also requires at least 50% loading of the coating pigment and does not recognize recyclability or repulpability.
In view of the fact that the prior art has still not developed an acceptable technology for producing repulpable waxed-based coated containers or the like, a need still exists for improvements in this area of technology. Responsive to this need, the present invention provides a method of making a repulpable material utilizing a hot melt composition as well as a method of repulping such coated articles. Broadly, the present invention uses an inorganic mineral filler in combination with a hot melt formulation as a coating which is both moisture resistant and repulpable when applied to a fibrous substrate. More preferably, a clay material is used as the inorganic mineral filler.
The prior art has not taught or suggested a repulpable coated article, a method of making such an article or a method of repulping according to the instant invention.
It is a first object of the present invention to provide a moisture resistant paper product which can be repulped.
Another object of the invention is to provide a repulpable paper product that can be recycled, particularly into unblemished sheets.
A still further object of the present invention is to provide a method of coating to form a moisture resistant paper product, which does not require corrosion resistant coating equipment so that existing equipment can be used.
Yet another object of the present invention is to provide a moisture resistant coating which is transparent, repulpable and meets FDA approval for food contact.
In its broadest sense, the present invention is directed to a coated article comprising a fibrous substrate and a coating consisting essentially of a hot melt formulation having an effective amount of an inorganic mineral filler, the coating being applied to the fibrous substrate to form the coated article with an exposed moisture resistant surface, the effective amount of inorganic mineral filler making the coated article repulpable. As part of the invention, the coated article, after being used, can be repulped and, if desired, recycled.
In another aspect of the invention, a method of making the coated article is disclosed wherein a fibrous substrate is provided and a coating consisting essentially of the hot melt formulation having the effective amount of the inorganic mineral filler therewith is applied to at least one surface of the fibrous substrate to form the coated article having both the exposed moisture resistant surface and repulpability.
In its broadest sense, the coated article can take any form including a sheet form, container form or the like. The fibrous substrate can be any fibrous substrate known for use in these types of applications, including paper, paperboard, kraft, non-woven materials, cellulosic-containing materials or the like.
The inorganic mineral filler is defined as a powdered substance that is mixed or dispersed in the hot melt formulation in which it is relatively insoluble, the dispersion of the hot melt formulation with the inorganic mineral filler remaining stable at elevated temperatures, e.g. 93xc2x0 C. for a minimum of 7 days, and not changing color over time, for example, 7 days at 93xc2x0 C. The inorganic mineral filler should also be able to be completely and uniformly dispersed in the hot melt formulation so that the coated article with the coating applied thereto is both moisture resistant and repulpable.
More preferably, the inorganic mineral filler is a clay such as a kaolin clay, calcined clay, ball clay, smectite clays such as montmorillonite, hectorite, or the like and chemically structured clays, e.g., clay-based pigments such as Sampaque(trademark), a chemically structured clay derived from the hydrothermal reaction of clays and sodium silicates as made by J. M. Huber Corp., Wrens, Ga., Norplex(trademark), a chemically structured clay/TiO2 composite pigment as made by Nord, Jeffersonville, Ga., Exsilon(trademark), a chemically structured clay combining clay and catonic polymers as made by Engelhard Corp., or the like.
The hot melt formulation can be any hot melt formulation which, when combined with the inorganic mineral filler in the effective amount, achieves the desired moisture resistance and repulpability in the coated article. Preferably, the hot melt formulation is a polymer-wax formulation as defined below.
The term xe2x80x9chot melt formulationxe2x80x9d as used in the present application means a thermoplastic formulation applied to the surfaces of paper products in a molten state, then allowed to cool.
The term xe2x80x9ccoatingxe2x80x9d as used in the present application means a method of applying a hot melt formulation to paper products without solvents or other carriers using any of the conventional techniques mentioned in Table III below.
The wax or waxes used in accordance with the present invention include a wide variety of waxes and resins. Such waxes include animal, vegetable and mineral waxes such as paraffin and microcrystalline, as well as synthetic waxes. As used herein, the term xe2x80x9cwaxxe2x80x9d shall include all such waxes and such resins used in hot melt coating compositions known in the art.
More preferably, the clay as the inorganic mineral filler ranges between about 5 and about 55 weight percent of the total coating composition. All references to the weight percentage of the inorganic mineral filler are based on the weight of the total coating composition. The percentage of the clay or other inorganic mineral filler also depends on the type of coating process utilized when applying the coating. For example, using curtain coating processing, the inorganic mineral filler may range between about 30 and less than 50%. Using an impregnation type coating, about 20 to about 30% of the inorganic mineral filler may be acceptable to achieve moisture resistance and repulpability.
Preferably, when the coated article is being repulped, the repulping operation is performed under neutral pH conditions. Repulping the coated articles using neutral pH conditions permits the use of existing equipment and processes.
In the method of making the coated article, the hot melt formulation and inorganic mineral filler are first mixed to form a dispersion of the hot melt formulation containing the effective amount of the filler. This inorganic mineral filler-containing formulation is then applied to the surface of a fibrous substrate in a conventional manner to form the coated article having both moisture resistance and repulpability.
Since the coated article according to the invention is repulpable, after its use, the coated article can be repulped and the repulped fibers can then be recycled using conventional technology.